1. Field of the Invention
This invention relates in general to spin valve magnetic transducers for reading information signals from a magnetic medium and, in particular, to a spin valve sensor with a pinned keeper layer, and to magnetic storage systems which incorporate such sensors.
2. Description of Related Art
Computers often include auxiliary memory storage devices having media on which data can be written and from which data can be read for later use. A direct access storage device (disk drive) incorporating rotating magnetic disks is commonly used for storing data in magnetic form on the disk surfaces. Data is recorded on concentric, radially spaced tracks on the disk surfaces. Magnetic heads including read sensors are then used to read data from the tracks on the disk surfaces.
In high capacity disk drives, magnetoresistive (MR) read sensors, commonly referred to as MR sensors, are the prevailing read sensors because of their capability to read data from a surface of a disk at greater track and linear densities than thin film inductive heads. An MR sensor detects a magnetic field through the change in the resistance of its MR sensing layer (also referred to as an "MR element") as a function of the strength and direction of the magnetic flux being sensed by the MR layer.
The conventional MR sensor operates on the basis of the anisotropic magnetoresistive (AMR) effect in which an MR element resistance varies as the square of the cosine of the angle between the magnetization in the MR element and the direction of sense current flowing through the MR element. Recorded data can be read from a magnetic medium because the external magnetic field from the recorded magnetic medium (the signal field) causes a change in the direction of magnetization in the MR element, which in turn causes a change in resistance in the MR element and a corresponding change in the sensed current or voltage.
Another type of MR sensor is the giant magnetoresistance (GMR) sensor manifesting the GMR effect. In GMR sensors, the resistance of the MR sensing layer varies as a function of the spin-dependent transmission of the conduction electrons between magnetic layers separated by a non-magnetic layer (spacer) and the accompanying spin-dependent scattering which takes place at the interface of the magnetic and non-magnetic layers and within the magnetic layers.
GMR sensors using only two layers of ferromagnetic material (e.g., Ni--Fe) separated by a layer of non-magnetic material (e.g., copper) are generally referred to as spin valve (SV) sensors manifesting the SV effect. In an SV sensor, one of the ferromagnetic layers, referred to as the pinned layer (reference layer), has its magnetization typically pinned by exchange coupling with an antiferromagnetic (e.g., NiO or Fe--Mn) layer. The pinning field generated by the antiferromagnetic layer should be greater than demagnetizing fields (about 200 Oe) at the operating temperature of the SV sensor (about 120 C.) to ensure that the magnetization direction of the pinned layer remains fixed during the application of external fields (e.g., fields from bits recorded on the disk). The magnetization of the other ferromagnetic layer, referred to as the free layer, however, is not fixed and is free to rotate in response to the field from the recorded magnetic medium (the signal field). In the SV sensor, the SV effect varies as the cosine of the angle between the magnetization of the pinned layer and the magnetization of the free layer. Recorded data can be read from a magnetic medium because the external magnetic field from the recorded magnetic medium (the signal field) causes a change in direction of magnetization in the free layer, which in turn causes a change in resistance of the SV sensor and a corresponding change in the sensed current or voltage. IBM's U.S. Pat. No. 5,206,590 granted to Dieny et al., incorporated herein by reference, discloses a GMR sensor operating on the basis of the SV effect.
FIG. 1 shows a prior art SV sensor 100 comprising a free layer (free ferromagnetic layer) 110 separated from a pinned layer (pinned ferromagnetic layer) 120 by a non-magnetic, electrically-conducting spacer layer 115. The magnetization of the pinned layer 120 is fixed by an antiferromagnetic (AFM) layer 130.
FIG. 2a shows another prior art SV sensor 200 with a flux keepered configuration. SV sensor 200 is substantially identical to the SV sensor 100 shown in FIG. 1 except for the addition of a keeper layer 206 formed of ferromagnetic material separated from the free layer 110 by a non-magnetic, spacer layer 208. The keeper layer 206 provides a flux closure path for the magnetic field from the pinned layer 120 resulting in easier pinned layer saturation and reduced magnetostatic interaction of the pinned layer 120 with the free layer 110. U.S. Pat. No. 5,508,867 granted to Cain et al., incorporated herein by reference, discloses an SV sensor having a flux keepered configuration.
FIG. 2b shows a perspective view of the SV sensor 200. This view shows the multilayer structure of SV sensor 200 as a ribbon-like sheet (stripe) extending away (downward in FIG. 2b) from the air-bearing surface (ABS). The keeper layer 206 is formed as a keeper layer stripe 260 having a front edge 270 at the ABS and extending away from the ABS to a rear edge 272. In SV sensors having a flux keepered configuration, the keeper layer 206 maintains its magnetization due to magnetostatic interaction with the pinned layer and to sense current induced fields. These forces are generally sufficient to properly orient the magnetization of a central portion 280 of the keeper layer stripe 260 as indicated by arrows 282, but usually leave a substantial region at the front and rear edges 270, 272 of the keeper layer stripe 260 with magnetization canted relative to the central portion 280 as indicated by arrows 274, 276. This means that only the central portion of the keeper layer stripe is providing flux keepering. In order to completely cancel the pinned layer moment a thicker keeper layer must be used. The thicker keeper layer increases the amount of sense current shunted through the keeper reducing the magnetoresistance signal. The magnetization in the canted regions at the front and rear edges 270, 272 of the keeper layer stripe 260 can also rotate in the presence of a signal field, thus shunting flux away from the free layer 110 resulting in a smaller detected signal.
Therefore there is a need for an SV sensor that provides improved orientation of the magnetization of the keeper layer in order to enhance the effectiveness of the keeper layer to form a flux closure path for the pinned layer magnetization.